Location information on a mobile station is needed in mobile communication systems for routing of calls and for other services. FIG. 1 in the attached drawings shows a simplified example of the structure of a GSM mobile communication system as a block diagram. The units shown in the Figure are also found in other mobile networks, but may have different names. A mobile station MS in standby or active mode is in continuous connection with a Base Transceiver Station BTS. A Base station subsystem BSS comprises a Base Station Controller BSC and its subordinate base stations BTS. Usually a plurality of base station controllers BSC are subordinate to a Mobile Services Switching Centre MSC. The Mobile Services Switching Centre MSC switches messages to other mobile services switching centres. Location information on a mobile station MS and other subscriber data are in permanent storage in a Home Location Register HLR of the system and temporarily in the Visitor Location Register VLR in whose area the mobile station MS is located at each particular moment. The visitor location register VLR comprises subscriber data required for all mobile stations MS in the area of the VLR.
The geographical area supervised by a visitor location register is divided into one or more Location Areas LA1 to LA3, within each of which a MS can move freely without notifying the VLR. One or several base stations BTS may operate in each location area. Base stations BTS continuously transmit information on themselves and their environment, such as a Base Statior Identity Code BSIC and a Location Area Identifier LAI. On the basis of the latter, a mobile station MS locked to a base station STS knows in which particular location area LA it is located. When the base station BTS is being changed and the mobile station MS detects that the location area identifier LAI of the base station has changed, the mobile station sends a location update request to the network. The location area of the mobile station MS is updated in the visitor location register VLR within whose area the mobile station is located at each particular moment. Information on the VLR within whose area the MS is located is transmitted to the home location register HLR.
A visitor location register VLR may lose the subscriber data due to VLR failure, software update, or restart following equipment maintenance. To recover from this, the visitor location register VLR has to request update of necessary subscriber data from the home location register HLR. Accurate location information can, however, be received from mobile stations only. In this case a mobile station MS has to be paged in all location areas LS of the visitor location register VLR involved. If a visitor location register serves a large number of subscribers, paging a mobile stations causes significant overload mainly in three sectors: 1. between the mobile services switching centre MSC and the base station subsystem BSS, 2. in the base station subsystem BSS signalling process, and 3. on radio channels. Location information is not updated until the data are needed, i.e. when service has to be offered to a mobile subscriber, e.g. in case of an incoming call, or when the mobile station establishes connection in order to make a call or to update its location. In practice the location update often occurs simultaneously for each MS. Prior art systems cannot prevent overload in a mobile system in this situation. Normal network operation can also cause overload in a mobile system if an exceptionally large number of subscribers is located in a particular location area LA, and they load the base station subsystem BSS with calls and location updates.